The concept and classification of "bars"

1) The concept of "bar"

On the lithographic prints, there are banded imprints in the direction parallel to the axis of the drum, resulting in irregular shades of ink due to irregular deformation of the dots, also known as "bars" and "strip marks."

2) Classification of "bars"

According to the "bar" of the ink can be divided into two kinds of ink bars and water bars.

In the print area of ​​the printed matter, the dots are irregularly enlarged, and the ink layer is thicker than the ink printed at the same time. There is a dark banded print (also called a "color bar").

Plumbing: In the printed area of ​​the printed matter, the dots are irregularly reduced, and the ink layer is thinned, which is significantly lighter than the ink printed at the same time (also called "white bars").

According to "bars," the causes can be divided into gear bars, vibration bars, roller bars and other bars.

Gear bars: The accuracy of the transmission gears of the cylinders, in-transit components, and water delivery components of the offset press is insufficient, resulting in wear of the tooth surfaces or damage of the gear teeth. Vibrators are generated in the meshing drive, causing the printed images on the rollers to slip relative to each other. Move to "bar". It is characterized by the fact that the pitch of the bars in the printed sheets is equal to the pitch of the gears, and the layout of the sheets is similar to that of the "cooked board."

Vibrating bar: The offset machine suddenly vibrates at a moment when it is running at a constant speed. This vibration destroys the drum to rotate at a constant speed and causes a slight amount of shaking between the printing rollers to create a "bar" printing market. Such as the printing roller face and the neutral shift, roller clutch, the opening and closing of the roller bite teeth, the inertia of the cam link mechanism or bearing wear and other transmission parts, will cause shock vibration, resulting in relative slippage of printed graphics A "bar" appears. It is characterized by the presence of “bars” in the local position of the printed sheet and its position is fixed. The position of the “bar” is related to the periodic and regular vibration of the mechanical movement.

Roll bar: The roller bar is due to the uneven ink design of the offset press, the unsuitable design of the uniform water system, or the accuracy of the roller processing, and the improper matching accuracy, which causes the slippage between the water and ink rollers and the plate cylinder and roller during operation. Or, due to improper pressure adjustment, the contact position is recessed, disturbing the normal transfer of ink and ink, thus forming a "bar" in central China. It is characterized by a slightly larger number of bars, a wider belt, a bar width equal to the surface perimeter of the roller, and an unfixed position. The inks are of different colors before and after printing.

Other bars: Bars generated by factors such as blankets, pads, printing plates, paper, inks, wetting fluids, and handling processes involved in the printing process. Because of its different characteristics, this article will not be discussed in detail.

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